1. Field of the Invention
The present invention relates to a lens device and is particularly suitable for application to a lens device of a surveillance camera.
2. Description of the Related Art
Surveillance cameras (CCTV cameras for surveillance purpose) are widely used in branches of a financial institution or shops for selling merchandise with a view toward preventing crime. A high-sensitivity surveillance camera capable of capturing an image at a low light level is recently put into practical use.
When a high-luminance subject is photographed, an amount of light radiated on an imaging element of the surveillance camera must be reduced to a small aperture. To this end, a diaphragm is set to a smaller value, or there is performed operation for decreasing the speed of an electronic shutter of the imaging element. However, when the diaphragm of the lens is reduced, a diffraction phenomenon occurs in a light ray passing through the diaphragm, which in turn deteriorates an image. Therefore, there is adopted a technique for attaching an ND filter to a diaphragm vane, to thus reduce the amount of light and make an opening of the aperture wider.
However, when the ND filter is attached to the diaphragm vane, the amount of light acquired when a high-luminance subject is photographed can be reduced. However, the amount of light is also decreased during opening of the diaphragm, which hinders photographing of a low-luminance subject and raises a problem of an increase in the minimum illuminance of the subject. In the meantime, in order to decrease the minimum illuminance of the subject, it is desirable that the density of the ND filter should be decreased. However, the amount of light cannot be decreased sufficiently when a high-luminance subject is photographed. Therefore, there arises a necessity for reducing the aperture of the diaphragm to a much greater extent. For this reason, there exists a problem of deterioration of image quality being induced by the diffraction phenomenon. In particularly, in the case of a supersensitive camera, an increase in the density of the ND filter poses difficulty on the camera to perform photographing at a dark place, which is the original merit of the camera.
When the speed of an electronic shutter is increased to reduce the amount of light during photographing of a high-luminance subject, the imaging element remains exposed to light of high luminance at all times rather than undergoes light attenuation caused by the diaphragm or the ND filter. Therefore, when the high-luminance subject is photographed by increasing the speed of the electronic shutter, a phenomenon (smear)—appearance of linear light on a photographed image—is likely to arise, which raises a problem of deterioration of image quality.
When a high-luminance subject is photographed by means of a supersensitive camera, the shutter speed must be increased further, and a critical shutter speed which is faster than the shutter speed of a normal camera is readily achieved. Therefore, there arises a problem of smear becoming more likely to arise. Even when the amount of light is reduced by means of the diaphragm, the aperture of the diaphragm must be reduced further because the camera is highly sensitive. Therefore, a diffraction phenomenon of the diaphragm becomes more likely to arise, which leads to a problem of deterioration of an image.